The present invention relates generally to the treatment of orthopedic implant devices, and more particularly, to a surface treatment applicable to such devices in which the wear resistant properties of orthopedic implant devices are enhanced while the fatigue strength is maintained.
More specifically, the present invention relates to orthopedic implant devices which are often used by surgeons to replace or repair bones and joints which have either been fractured or have degenerated. Examples of such orthopedic implant devices are disclosed in U.S. Pat. No. 4,864,608 which describes a hip prosthesis in which the surface treatment process of the present invention is applicable. The present invention may also be applicable to other orthopedic implant devices such as knee and shoulder prostheses, as well as other types of orthopedic implants which are subject to wear and fatigue.
Representative of the functions of many different types of orthopedic implant devices, the hip prosthesis disclosed in U.S. Pat. No. 4,864,608 is used to replace a natural hip which has degenerated. The hip prosthesis includes a stem which is inserted into and supported by the femur, as well as a projecting ball which is designed to fit within the acetabular socket of the patient receiving the implant. Because the hip prosthesis is under relatively high stress during usage, the hip prosthesis may experience wear for a variety of reasons.
It is generally known that surface treatment of orthopedic implant devices may improve wear characteristics. For example, ion implantation is used to place a nitrogen-enriched layer on the surface of orthopedic implant devices. Because such orthopedic implant devices are typically formed from a titanium-based material, the resulting titanium nitride layer creates a wear resistant surface layer. Physical vapor deposition is also used to create a wear resistant layer on orthopedic implant devices. When using physical vapor deposition, metal is evaporated and then condensed on the surface of the orthopedic implant device so as to form a wear resistant coating.
While these processes are generally successful in improving the wear resistance characteristic of orthopedic implant devices, such processes often have several disadvantages. For example, the ion implantation technique is effectively a "line of sight" procedure which makes it difficult to treat portions of orthopedic implant devices which have relatively irregular contours. When physical vapor deposition is used, the resulting titanium nitride coating may not be sufficiently supported by the substrate so that the coating may be relatively easily damaged when structure underlying the layer is subject to minor deformation. Furthermore, if the coating is cracked due to deformation of the underlying substrate, the crack may tend to propagate into the underlying substrate. Finally, because both ion implantation and physical vapor deposition techniques use relatively small batch sizes and are costly processes, the per unit cost of treating orthopedic implant devices using these techniques is relatively large.
While not generally used with orthopedic implant devices, plasma diffusion is also a known method for improving the wear characteristics of materials. In such a process, the material undergoing plasma diffusion is heated to relatively high temperatures (approximately 900.degree. C.). However, this process is presumably believed to be undesirable for use in orthopedic implant devices for several reasons.
First, plasma diffusion is believed to reduce the fatigue strength of the material from which the orthopedic implant device could be formed. Second, the typical plasma diffusion technique often uses thermocouples to control processing. Such thermocouples are often located within the body of the article to be treated and therefore may not provide a relatively accurate indication of the temperature of the outer surface of the article per se. This is of particular concern with titanium-based articles which are not generally good thermal conductors and therefore the temperature at the surface of the article may be much different than the temperature at a position below the surface. Because such thermocouples are often not capable of accurately determining the surface temperature of article being processed, plasma diffusion processing often did not produce consistently uniform wear resistant surface layers on titanium.